This invention relates to a proximity detector, and especially to a ferrous-gear-tooth Hall-transducer, or other magnetic-field-to-voltage transducer, capable of detecting the leading and wailing gear tooth edges of an adjacent rotating ferrous gear, or other magnetic articles, and more particularly relates to such a Hall proximity detector with an automatic gain adjust feature in the Hall-voltage amplifier
The term "magnetic article" as used herein applies to magnetized bodies, ferrous bodies and other bodies having a low magnetic reluctance that tend to alter the ambient as magnetic field.
In the U.S. Pat. No. 5,442,283, issued Aug. 15, 1995 there is described a Hall-voltage slope-activated proximity-detector capable of detecting the rising and falling edges of an adjacent rotating gear tooth. This proximity-detector type detector includes an integrated circuit Hall detector mounted to a pole of a magnet, and includes a circuit for tracing a slope of a Hall voltage (e.g. corresponding to the approach of a passing gear tooth) and briefly holding the ensuing peak voltage before producing an output signal indicating the onset of the following Hall-voltage slope of opposite direction (e.g. corresponding to the approach of a valley between two gear teeth). The Hall voltage holding circuit includes a capacitor and circuit means for controllably leaking charge out of or into the capacitor for preventing false tripping of a comparator that provides the pulse output signal.
The holding voltage of the capacitor thus has a droop which leads to increasing loss of holding accuracy as the speed of gear tooth passage becomes slower, and therefore the detector has a minimum gear teeth speed at which accurate detection is possible.
Most proximity detectors of the prior art produce a high binary output voltage indicating approach and proximity of a passing article, and produce a low binary voltage when the article recedes from the detector. The transition in detector output voltage from low to high typically is triggered by a comparator that determines when the transducer voltage rises to a fixed internal threshold voltage reference. Alternatively, in the case of the above described slope-activated detector, the detector determines when a transducer voltage peak has just occurred and the transducer signal voltage drops a predetermined incremental voltage from the peak value.
Prior art proximity detectors having fixed threshold voltages, produce low to high (or high to low) binary transitions in the output signal indicating approach of a magnetic article. In practice, the closest passing distance (sometimes referred to as the air gap) does not remain constant.
Variations of the air gap dimension causes shifts in the actual distances of article approach and receding at which the transducer voltages exceeds or falls below the fixed thresholds. This results in a lack of accuracy of passing detection that may rule out their use as position detectors of passing articles such as cams and gear teeth.
Changes in the air gap, between passing articles to be detected and the transducer, may be attributable to mechanical and electrical properties of the detector as well as in the properties of the passing articles, especially as a function of temperature.
The result is a detection inaccuracy that may rule out the use of such detectors for such critical applications as in combustion-engine ignition distributors. Prominent causes of this inaccuracy stem from the fact that the amplitude of the Hall voltage changes when gear teeth (articles) have different ferro-magnetic properties from tooth to tooth, and/or when undulating changes in the spacings (air gap) of gear teeth to detector are caused by eccentricity of the gear. Also, changes in temperature cause changes in air gap dimensions and in the sensitivity of the transducer and transducer-voltage amplifier.
Whether detection is accomplished by sensing the Hall voltage peaks or using a voltage threshold criteria for indicating approach of a passing article, changes in the median amplitude of the transducer voltage degrade the accuracy of position detection.
Over multiple installations, the effective air gap to which a transducer in the proximity detector may be subject can vary by several millimeters. At a relatively wide air gap, the amplitude of the peak to peak signal generated by the transducer such as the Hall device is many times less than the amplitude of the same signal at a relatively close or narrow air gap. To provide an electrical signal having a substantially constant peak to peak signal amplitude over a majority of the air gap range, the proximity detectors employ automatic gain control (AGC).
With AGC, the gain can be opted for each air gap to which the proximity detector may be subject after power up. At relatively narrow air gaps, the AGC minimizes the gain to ensure that a magnetic signal having a relatively large amplitude will not result in clipping or other distortion of the electrical signal generated by the transducer. At relatively wide air gaps, the AGC maximizes the gain to thus allow processing of electrical signals generated from a magnetic signal having a relatively small amplitude. Thus, by using AGC, the a proximity detector can operate over a relatively wide range of air gaps. Moreover, by providing an electrical signal having a substantially constant peak-to-peak signal amplitude regardless of air gap width, the proximity detector provides improved timing accuracy over the entire range of air gap widths.